1. Field of the Invention
The present invention relates to a sol-gel process, and in particular, to a method for fabricating a silica glass monolith containing high-purity and high-density silica, for use in fabricating an optical fiber forming glass.
2. Description of the Related Art
In general, many methods have been suggested to fabricate a glass monolith by a sol-gel process. Example of some sol-gel processes are found in U.S. Pat. No. 4,419,115 to David W. Johnson, et al., entitled Fabrication Of Sintered High-Silica Glasses, discusses a two-step gelling process for the fabrication of sintered high-silica glass wherein fumed silica is mixed with deionized water to form a first gel not in need of pH adjustment. This first gel is then dried, wherein the drying may occur in a humidity controlled environment. The dried first gel is thermally treated to raise the temperature of the dried first gel in the vicinity of 750 to 850 degrees Celsius and then cooled, then the cooled dried first gel is then fragmented, redispersed in deionized water and placed in a closed mold to form a second gel, which is then dried and sintered to form a glass monolith. Also described therein is a pH adjusting step of adding boric acid to the deionized water to decrease the pH which lowers the gelling rate and that a further decrease in pH will increase the gelling rate. U.S. Pat. No. 4,605,428 to David W. Johnson, et al., entitled Sintered High-Silica Glass And Articles Comprising Same discusses some modification of the two-step gelling process for the fabrication of sintered high-silica glass of the U.S. Pat. No. 4,419,115 patent. U.S. Pat. No. 4,624,800 to Yutaka Sasaki, et al., entitled Method For Producing Aqueous, Low-Alkali Metal, Low-Alumina Silica Sols, describes a method for producing an aqueous silica sol by mixing purified water and a silica powder. The pH of the sol is adjusted by addition of an acid and/or a base (ammonia). U.S. Pat. No. 4,680,045 to Haruo Osafune, et al., entitled, Method Of Preparing Tubular Silica Glass, describes a sol-gel method of preparing tubular silica glass using a mixture of silicon alkoxide, water, hydrochloric acid and fumed silica to form a sol, which is gelled, dried and sintered to form the glass. Adjusting the pH of the sol is discussed in col. 4, line 63-col. 5, line 15. U.S. Pat. No. 4,680,046 to Nobutaka Matsuo, et al., entitled, Method Of Preparing Preforms For Optical Fibers, describes a sol-gel process for forming preforms for optical fibers wherein the pH of one sol solution was adjusted by the addition of ammonia water and water. Another sol solution was prepared by including alcohol (anhydrous ethanol); and this sol solution had the pH adjusted by the addition of hydrochloric acid and then readjusted by the addition of ammonia water and water. U.S. Pat. No. 4,681,615 to Motoyuki Toki, et al., entitled, Silica Glass Formation Process, describes a process for forming monolithic silica glass by mixing hydrochloric acid, ethyl-silicate and fumed silica (and in some examples water was added) to form a sol having its pH adjusted by the addition of an ammonia solution. The sol was then gelled in a cubical container, dried and sintered to form the silica glass. U.S. Pat. No. 4,786,302 to Haruo Osafune, et al., entitled, Method Of Preparing Tubular Silica Glass, describes a sol-gel method of preparing tubular silica glass using a mixture of silicon alkoxide, water, hydrochloric acid and fumed silica to form a sol, which is gelled, dried and sintered to form the glass, wherein the pH of the sol is adjusted. U.S. Pat. No. 5,250,096 to Allan J. Bruce, et al., entitled, Sol-Gel Method Of Making Multicomponent Glass, describes forming a first sol using a mixture of Si-alkoxide, alcohol (e.g. ethanol) and one or more metal compounds added to an appropriate liquid (alcohol if the compound is an alkoxide or an aqueous medium if the compound is a soluble salt). The first sol is gelled at 60 degrees Celsius, broken down to a fine powder and dried at a temperature between 60 and 150 degrees Celsius. The powder is redispersed in water, having its pH adjusted by the addition of an acid, to form a second sol, cast in molds of glass tubing and rubber stoppers to gel. The gel is removed from the mold, dried and sintered to form a low alkali silica glass. The addition of ammonia water to raise the pH is also described therein. U.S. Pat. No. 5,254,508 to Fikret Kirkbir, et al., entitled, Sol.sub.-- Gel Process For Forming A Germania-Doped Silica Glass Rod, describes a sol-gel process for forming germania doped silica class rods utilizing alkoxides, deionized water, ethanol and hydrofluoric acid.
Silica glass obtained by using fumed silica powder only is vulnerable to cracking during a drying step, thus it fails to achieve its general use. In a sol-gel process using silicon alkoxide, a glass body is homogeneous and transparent but has a very high shrinkage rate (i.e, higher than 60%), which makes it difficult to use the glass body for a large secondary glass tube for fabrication of an optical fiber.
In a process using fumed silica particles, wherein a first sol is formed by dispersing the fumed silica particles in water to prevent cracking, gelled, dried, and powdered, and a second sol is formed by thermally treating the silica powder and re-dispersing the thermally treated silica powder in water, gelled, dried, and sintered, so that the thus-obtained powder particles are larger than the original powder particles, the size of pores among the particles is increased, and a crack-free silica glass monolith is obtained. When fabricating a large glass monolith in the above process, even the two sol dispersion steps cannot prevent cracking during the drying steps, and a long time is required to gel the first sol.